Method of producing refractory metals and refractory metal compounds in powder form

ABSTRACT

REFRACTORY METALS ARE PRODUCED USING A GASEOUS HALIDE PROCESS IN WHICH THE PROCESS IS ACCELERATED BY EMPLOYING CRYSTALLIZATION SEEDS IN THE REACTION.

US. Cl. 75-.5 BB 1 Claim ABSTRACT OF THE DISCLOSURE Refractory metalsare produced using a gaseous halide process in which the process isaccelerated by employing crystallization seeds in the reaction.

This is a division of application Ser. No. 145,922, filed Mal 21, 1971,now U.S. Pat. No. 3,723,601.

The present invention relates to a method of producing a powder of oneof the refractory metals V, 'Nb, Ta, Cr, Mo, W and Re or a mixture oftwo or more of these metals. The invention also relates to theproduction of carbides, borides or nitrides of one of the metals Ti, Zr,Hf, V, Nb, Ta, Cr, Mo and W, or a mixture of two or more of these metalsfrom the halide(s) of the metals in the gaseous phase.

Thus, broadly speaking, the present invention relates to powderpreparation. The grain size of powders of certain metals and metalcompounds can thus be controlled, as the powder is prepared from thehalides of the corresponding metal in the gaseous phase. In certaincases also the shape or form of the grains can be influenced.

Powders of these refractory metals or metal compounds are usuallyprepared in batches. Thus, for instance, tungsten powder is preparedfrom a batch of tungsten oxide by reducing the oxide with hydrogen gaspassing through the furnace. Titanium carbide can also be prepared byreacting titanium dioxide and carbon in batches. In commercial practice,a grain size of more than 1 m. is generally obtained. However, there isa need of powders of a smaller grain size. According to the literature,processes are known for the production of, for instance, tungstencarbide powder having a smaller grain size, but the production costs arehigh and, therefore, the proceszes lack commercial interest.

Other methods for the preparation of such powders include those startingfrom metal halides in the gaseous phase. Thus, the metal powder may beprepared by reacting a metal halide in the gaseous phase with hydrogengas; metal carbides may be prepared by carrying out the reaction withhydrogen gas in the presence of a gascontaining carbon; while metalborides or nitrides can be prepared in analogous manner. When suchprocesses are used for the preparation of powder, the powder is often sofinely divided that, for this reason, it has limited use.

Some additional difficulties occur in ga phase reactions between thehydrogen gas employed and metal halides. It has often turned out to bevery difficult to obtain a major amount of powder freely precipitated inthe reactor. The metal or the metal compounds tend to deposit on thewalls of the reactor or on objects in the reactor, which deposit resultsin the formation of films or flakes, which gradually come loose and falldown in the powder formed. In this way, the composition of the productbecomes non-uniform and the product cannot be used directly. An inferioryield is achieved in the reaction and the reactor must now and then bestopped for removal of the coatings. This precipitation on the reactorwalls can be inhibited by raising the reactor temperature, but

3,832,157 Patented Aug. 27, 1974 the result of this is a powder that isstill more finely divided.

In several cases, the reaction in the gas phase is very slow and onlyvery small amounts of powder are obtained.

This is, for instance, the case in preparing carbides of the metalstitanium, zirconium and hafnium. In such cases, it has been necessary toresort to reduction under the influence of a hydrogen plasma. Thisprocess is, however, expensive and provides extremely fine powder.

These difficulties are substantially eliminated by employing the presentprocess, and, furthermore, the reaction in the gas phase is acceleratedand a powder is obtained instead of deposits on the reactor walls. Thegrain size of the powder can moreover be influenced by the presentprocess. The present process is characterized in that the reaction iscarried out in the presence of crystallization seeds or seed-formingsubstances.

It is known to promote the crystallization in solutions by addition ofseeds. Sugar solutions can, for instance, be brought to crystallizationby adding a fine sugar powder to a saturated sugar solution. By means ofcontinued evaporation, crystallization of uniform sugar crystals isobtained.

Another example of known art is production of titanium dioxide fromsulphate solution by addition of colloidal crystallization seedsinfluencing the reaction, the yield and the size and shape of thecrystals.

According to the present invention, it has now been surprisinglyobserved that it is possible to influence such reactions in the gaseousphase whereby powders of the metals V, Nb, Ta, Cr, Mo, W and Re orpowders of carbides, borides or nitrides of the metals Ti, Zr, Hf, V,Nb, Ta, Cr, M0 and W are obtained by addition of crystallization seeds.Apparently, the reactions take place on the surface of these seeds,which results in a more rapid reaction. Slow reactions can in this waybe accelerated to proceed more completely. In cases where previouslyonly deposits on the reactor walls substantially could be obtained, thereactions can be brought to proceed more completely by the addition ofcrystallization seeds or seedforming substances, before the reactantsreach the reactor walls, By the present process, it is also possible tocontrol the grain size of the formed product by adjusting the amount ofcrystallization seeds or seed-forming substances relative to the amountof reactants. Coarser grains are obtained at less relative amounts ofcrystallization seeds and, on the other hand, a larger relative amountof crystallization seeds provides a finer grain size. In certain cases,it has also appeared to be possible to influence the shape or form ofthe grains, for instance, in the preparation of titanium carbide, whichcan be obtained either in the form of round grains or of well-shapedcrystals with plane surfaces.

The crystallization seeds can be added to the reactants or to thereactor in the form of a fine powder of the material to be prepared,i.e. powder of the metals or the metal compounds. It is also possible touse powders of some other material than that to be prepared in the caseswhere such contamination with foreign material is of no practicalimportance. Crystallization seeds can also be achieved by adding asubstance which will form crystallization seeds during the reaction.These substances are referred to herein as seed-forming substances.

Titanium carbide can be prepared by heating a mixture of titaniumtetrachloride, hydrogen gas and a hydrocarbon (e.g. methane) to reactiontemperature. This reaction takes place slowly and only minor amounts oftitanium carbide are received in the form of powder. -If small amountsof tungsten hexachloride or molybdenum pentachloride are added to thereaction as seed-forming substances, these materials react rapidlyforming metal or metal carbide in the form of fine grains, on thesurface of which the reaction to titanium carbide takes place veryrapidly. If tungsten and molybdenum are not desired as contaminants inthe titanium carbide powder, the reaction can still be brought tocontinue more rapidly by adding titanium tetraiodide, which substance ismore easily disintegrated to metal than titanium tetrachloride and whichthen serves as a seed-forming substance, i.e. forms crystallizationseeds of titanium.

It is, however, not necessary to add crystallization seed substances orseed-forming ones as such, but it is also possible to form or initiatesuch seeds in the reaction mixture. One can, for instance, add hydrogenas a seedinitiating gas preheated to reaction temperature in small buteffective amounts, the addition taking place continuously or by pulsesto the reaction so that locally conditions for a more rapid reaction tograins of metal or metal compunds are achieved, which grains serve ascrystallization seeds. It is also possible to bring locally the reactionmixture to considerably increased temperature to initiate the formationof seeds, for instance, by electrical discharges or sparks so that amore rapid reaction to grains of metal or metal compound takes placelocally, the grains also in this case serving as crystallization seeds,especially at the production of metal carbide powder. As an example ofsuch a carbonaceous compound, carbon tetrachloride can be mentioned.

The invention is described more closely in the following examples.

EXAMPLE 1 A mixture of TiCl CH H and WCl in the molar ratio l:1:5:0.009was introduced in gas phase into a reaction chamber which was maintainedat a temperature of 1300 C. From the titanium carbide formed at thereaction 92% were obtained in the form of a powder having a total carboncontent of 20.03%, of which free carbon was 0.50%.

When the reaction was carried out under the same conditions with thesame gas mixture which, however, did not contain a small but effectiveamount of WCl as seedforming substance, about of the titanium carbidewas obtained in the form of a powder.

EXAMPLE 2 The test in Example 1 was repeated but, instead of WCI a smallbut eflective amount of MoCl was used as seed-forming substance. 70% ofthe titanium carbide formed was obtained in the form of a powder. Thepowder contained totally 20.20% carbon, of which 0.55% was free carbon.

EXAMPLE 3 EXAMPLE 4 A gas mixture of TiCl C H H and WCl in the molarratio 1:1:10:0.009 was reacted in a reaction chamher at 1300" C.Titanium carbide was formed, of which was obtained in the form of apowder having a total carbon content of 19.64%; the content of freecarbon being 0.79%.

EXAMPLE 5 A mixture of TiCl NH H and WCl is provided in a molar ratio of1:1:2:0.01 which is introduced as a gaseous phase into the reactionchamber as in Example 1 at a temperature of about 1300 C. to formtitanium nitride. The small but effective amount of WC1 is employed asthe seed-forming substance.

EXAMPLE 6 A gaseous mixture of ZrC1 BC1 H and MoCl is introduced in themolar ratio of l:2:8:0.05 into the reaction chamber at a temperature ofabout 1400" C. to form zirconium boride, the small but effective amountof molybdenum pentachloride being used as the seedforming material.

EXAMPLE 7 Tantalum nitride is produced by feeding a gaseous mixture ofTaCl NH H and WCl in the molar ratio of 1:1:3:0.02 into the reactionchamber at a temperature of about 1200 C., the small but effectiveamount of WCI being employed as the seed-forming substance.

The foregoing examples illustrate the use of reactants selected from thegroup consisting of carbon-containing, boron-containing andnitrogen-containing materials in producing refractory metal compounds inpowder form.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claim.

What is claimed is:

1. In a process of producing a powdered metal product selected from thegroup consisting of V, Nb, Ta, Cr, Mo, W and Re by the reduction of agaseous halide of said metal in a reaction chamber, the improvementwherein said process is accelerated in the reaction chamber by means ofa seed-initiating reaction, which comprise injecting into the gaseoushalide a small but effective amount of preheated hydrogen heated to atleast the reaction temperature while feeding the gaseous halide into thereaction chamber.

References Cited UNITED STATES PATENTS 3,062,638 11/1962 Culbertson etal. --.5 BB 3,177,067 4/1965 Nichols 75-.5 BB 3,211,548 10/1965 Schelleret a1. 75-.5 BB 3,234,007 2/ 1966 Blocher et al. 75.4 BB 3,341,3209/1967 Smiley 75.5 BB

WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 75844, 84.5

